Many industrial and automotive devices require a pressurized supply of incompressible fluid such as lubricating oil to operate. Pumps, typically used to supply these fluids, can either be of variable or constant displacement designs.
With a constant displacement pump, the pump outputs a substantially fixed volume of working fluid for each revolution of the pump. To obtain a desired volume and/or pressure of the working fluid the pump must either be operated a given speed, independent of the speed of the automotive engine or other device supplied by the pump, or a pressure relief valve must be provided to redirect surplus flow, when the pump is operated above the speed required for the desired flow, to the low pressure side of the pump or to a working fluid reservoir.
With a variable displacement pump, the volumetric displacement of the pump can be altered, to vary the volume of fluid output by the pump per revolution of the pump, such that a desired volume of working fluid can be provided substantially independently of the operating speed of the pump.
Variable displacement pumps are typically preferred over constant displacement pumps with relief valves in that the variable displacement pumps offer a significant improvement in energy efficiency, and can respond to changes in operating conditions more quickly than pressure relief valves in constant displacement pumps.
A typical variable displacement pump is a single stage pump that includes a single spring. Prior art approaches to creating a multiple stage variable displacement pump have involved the addition of a solenoid that is controlled by the electrical system of the engine. Such prior art pumps are costly due to the requirement for electrical input to operate the solenoid incorporated in the pump.
U.S. Pat. No. 4,538,974 issued to Glyco Antriebstechnik GmbH, discloses a pump that has a stator formed with intake and output ports connected to the lubricating system of a internal combustion engine of an automotive vehicle for flow therethrough. The disclosed pump is directed to providing meaningful lubricant flow even when the lubricant is very viscous, normally when cold.
U.S. Patent Application Publication No. 2009-0101092 (corresponding to Japanese Patent Application Publication No. 2009-97424) discloses a conventional variable displacement pump including a first spring arranged to constantly apply an urging force to a cam ring, and a second spring arranged to provide an urging force in a direction opposite to the urging force of the first spring when the cam ring is moved by a predetermined distance or more. In this conventional variable displacement pump, an eccentric state of the cam ring is varied in two stages (steps) by the relative urging forces of the springs, so that the discharge flow rate characteristic is varied in the two stages.
U.S. Pat. No. 8,011,908 issued to Magna Powertrain Inc. discloses a variable capacity vane pump having a control ring that is moveable to alter the capacity of the pump. A primary spring acts between a control ring and the casing to bias the control ring towards a position of maximum volumetric capacity and a secondary return spring is mounted in the casing and is configured to engage the control ring after the control ring has moved a predetermined amount. This patent discloses springs that are not positioned to be inline and are not controlled by a pressure regulating spring guide.